This invention relates to a method for preparing low specific gravity silicone rubber elastomers having heat resistance.
Heat curing type liquid silicone rubber compositions are employed in a variety of applications since they are effectively moldable and exhibit good heat resistance, weather resistance and electrical insulation after molding. On the other hand, silicone rubber sponge is lightweight while maintaining the heat resistance, weather resistance, and electrical insulation inherent to silicone rubber. By virtue of the inclusion of gas in the molded part, the silicone rubber sponge serves the function of a shock absorber or cushioning member by utilizing its volume contraction and the function of a thermal insulating or thermal storage material by utilizing its low heat transfer, finding a wide variety of applications.
Such silicone rubber sponges are obtained by several methods, for example, by adding a thermally decomposable blowing agent, or by molding and curing while generating hydrogen gas by-product. In the method of adding a thermally decomposable blowing agent, the toxicity and odor of decomposed gases are problems. In the system wherein a platinum catalyst is used as the curing catalyst, the retardation of cure by the blowing agent is a problem. The method of utilizing hydrogen gas by-produced during the curing step suffers from such problems as the potential explosion of hydrogen gas and the careful handling of uncured composition during shelf storage. Further, the gas generating method encounters difficulty in forming controlled uniform cells particularly when the silicone rubber composition is a liquid one. One known approach addressing the above-mentioned problems is to incorporate hollow particles of inorganic material, typically glass or ceramic material into rubber. This method provides only a little contribution to weight reduction because of the high specific gravity and achieves only a little thermal conductivity decline because of the inorganic nature. It is also known to add a hollow filler of organic resin. Because of the poor heat resistance of the organic resin filler, the physical properties of molded parts largely vary with the lapse of time in the above-mentioned applications, failing to take advantage of the heat resistance of silicone rubber.
An object of the invention is to provide a simple method for preparing a low specific gravity silicone rubber elastomer or spongy silicone rubber having improved heat resistance, without giving rise to handling and other problems.
We have found that when a heat curable silicone rubber composition loaded with a hollow filler of organic resin is molded under such conditions that the hollow filler is not broken, and in the subsequent secondary curing step, the hollow filler is broken, there is obtained a low specific gravity silicone rubber elastomer having improved heat resistance. Since the silicone rubber is produced in the sponge state, the invention avoids changes of physical properties during service due to breakage of the hollow filler. The advantage becomes more outstanding when the composition is molded at a temperature below the softening point of the organic resin, and the hollow filler is broken in the secondary curing step which is effected at a temperature equal to or above the softening point of the organic resin; or when the composition is molded within 14 minutes at a temperature from the softening point of the organic resin to the softening point plus 20xc2x0 C., and the hollow filler is broken in the secondary curing step which is effected for at least 15 minutes at a temperature equal to or above the softening point of the organic resin.